The extended elastic impedance (EEI) concept has been used in the oil industry primarily for lithology and fluid prediction in exploration and development projects. We present a method of reservoir monitoring that identifies and maps changes in pressure and saturation in a producing reservoir by applying EEI to time-lapse seismic data. The method uses time-lapse seismic difference data rotated to specific EEI χ angles that are optimized for the changes expected in a given reservoir. One angle is found to be appropriate to identify predicted changes in saturation, using fluid-substitution models, while the other angle is found from rock-physics assumptions or laboratory measurements of fluid-pressure changes. Our technique is tested using time-lapse seismic data from the Enfield oil field in Australia's North West Shelf, with estimates of optimal EEI rotation angles χ based on log data and Biot-Gassmann modeling for the fluid changes (χ = +42°) and on rock-physics models fit to measurements made on core samples for the pressure changes (χ = –79°). Seismic reflectivity and inversion domains were used for comparison and analysis of the final rotated volumes. We used publicly available seismic data that had been recorded and processed in 2004 and 2007 when time-lapse processing was still being developed. The low reproducibility of these seismic data and the complexity of the reservoir architecture limited the extent of our interpretation and results. Nonetheless, our qualitative and quantitative results are encouraging and supported by field production data. This technically simple approach should be useful in the analysis of time-lapse seismic data processed by modern techniques and would help in the management of reservoirs using a straightforward and readily reproduced procedure.

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